1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, particularly to a reset circuit of a semiconductor integrated circuit which can prevent the malfunction of the semiconductor integrated circuit even when the operating voltage changes.
2. Description of the Prior Art
FIG. 1 is a circuit diagram of a conventional reset circuit of an integrated circuit having a microcomputer mounted thereon, and FIG. 2 is a graph showing change in power supply voltage.
If power source 21 is switched on and the voltage thereof exceeds the threshold level of N channel transistor 22, the drain voltage of N channel transistor 22 is lowered to ground level. Since the gate of N channel transistor 22 is connected to the drain of P channel transistor 23 and also to the drain of N channel transistor 22, the voltage of power source 21 is increased to exceed the sum of the threshold voltage of N channel transistor 22 and the threshold voltage of P channel transistor 23, then P channel transistor 23 also enters a conductive condition and functions to increase the drain voltage. Thereafter, when the voltage of the power source 21 increases, the drain voltages of the N channel transistor and P channel transistor increase in proportion to the ratio of the resistance of the N channel transistor to the resistance of the P channel transistor. If the drain voltage exceeds the logic threshold level of inverter 24 in the next stage, then the output of inverter 24 becomes ground level. In other words, inverter 24 outputs the voltage of the power source level from the time the voltage of power source 21 exceeds the sum of the threshold voltages of N channel transistor 22 and P channel transistor 23 until the time the drain voltage of the former stage exceeds the logic threshold level of inverter 24. The circuit operates in a manner entirely opposite to the above process when the power supply voltage drops. Therefore, by using the output of inverter 24 as a reset signal, the reset circuit is activated in a simple manner by voltage detection.
Another example of a conventional reset circuit is a reset circuit made of a one-chip microcomputer (disclosed in Japanese Patent Laid-open No. 221816/85) comprising, on a semiconductor substrate, an internal circuit element, a clock generation circuit for supplying a driving clock signal to the internal circuit element, a power terminal provided on the semiconductor substrate for connection with an external power source, a first power supply voltage detecting circuit, which receives power supply voltage from the power terminal, for detecting a state in which the power supply voltage drops to a value less than a first lower limit reference voltage Vs which guarantees the stable operation of the above internal circuit element and generates a clock generation stop signal for transmission to the above clock generation circuit, a second power supply voltage detecting circuit, which receives the power supply voltage from the power terminal, for detecting a state in which the power supply voltage is higher than the operative condition holding voltage Vr of the above internal circuit element but not more than a second lower limit reference voltage Vr which guarantees a stable operative condition of the above internal circuit element, for generating a reset signal, and setting means for setting the above internal circuit element to an initial state in response to the above reset signal. FIG. 4 is a block diagram of the one-chip microcomputer disclosed in Japanese Patent Laid-open No. 221816/85. FIG. 5 is a detailed circuit diagram which shows part of FIG. 4, including inverter 31, first and second power supply voltage detecting circuits 32, 33 and AND gate 34.
With reference to the conventional circuit shown in FIG. 1, the characteristic of manufacturing conditions are advantageously proportional to the characteristic line of the CPU operating voltage as shown in FIG. 3, that is, the manufacturing conditions of the circuit shown in FIG. 1 varies, from the view point of characteristic, in proportion to the CPU operating voltage characteristic line as shown in FIG. 3. Now, FIG. 3 will be described as follows. The relation between detected voltages of the conventional circuit and corresponding temperatures to which the circuit is subjected are shown in a graph of FIG. 3 as lines VTHHPOC and VTLLPOC, each having a negative temperature coefficient. Further, VTHH and VTLL represent a higher value and a lower value of the threshold level (hereinafter referred to as VT) of the transistor which are determined by manufacturing conditions. With VT of a low level, the detected voltage is low and with VT of a high level, the detected voltage is high. The CPU operating voltages have the characteristic as shown in the graph with lines VTHHCPU and VTLLCPU having a positive temperature coefficient. Further, the CPU lowest operating voltage is high as VT becomes high, and low as VT becomes low. Therefore, in trying to guarantee stable operation of the CPU in the range up to the reset voltage of the conventional reset circuit, within the range of guaranteed temperatures (for example, -40.degree. to +85.degree. C.), the detected voltage level of the conventional circuit exceeds the CPU lowest operating voltage level by a large margin when the temperature of the circuit is low. No matter how low the CPU lowest operating voltage is in practice, the CPU judges the detected voltage of the conventional circuit as the lowest operating voltage of the CPU and hence, from appearance, the detected voltage of the conventional circuit becomes the lowest CPU operating voltage. Accordingly, in order to meet the CPU lowest operating voltage in the whole range of the guaranteed temperatures, it becomes necessary to specify the voltage detected from the conventional circuit in the low temperature range as the CPU operating voltage in the worst conditions. However, with this arrangement, a voltage much higher than the real CPU lowest operating voltage is specified as the CPU lowest operating voltage.
In the Japanese Patent Laid-open No. 221816/85, a reference voltage is generated by resistances R7, R8, as shown in FIG. 5. However, it is difficult to correct the variations in the value of the resistance on the semiconductor substrate. In addition, as shown in FIG. 6, there is a difference between the oscillating voltage of an oscillator and the operating voltage of a microcomputer, and the detected operating voltage of the conventional circuit is lower than the CPU lowest operating voltage. Therefore, even if the operating voltage of the conventional circuit is detected after a failure of the CPU has occurred, it is difficult to make a correct judgment of the cause of the trouble, and hence it is impossible to detect a runaway of the microcomputer.